Typically, engines using gasoline or fuel containing gasoline as a major component widely adopt a spark ignition method of performing ignition by a spark plug. To improve fuel efficiency and the like, a technology has been developed in recent years, in which: a high compression ratio (for example, 14 or more) is applied as a geometrical compression ratio of the engine; gasoline or fuel containing gasoline as a major component is used; and in a predetermined operation range, compression self ignition (specifically, homogeneous-charge compression ignition (HCCI)) is performed.
A combustion chamber structure of the engine configured to perform the compression self ignition is disclosed in, for example, PTL 1. Regarding a combustion chamber structure applied to a high compression ratio engine, PTL 1 discloses a technology of improving filling efficiency by configuring the combustion chamber structure such that an inside of a cavity formed on a middle portion of a piston upper surface is adequately scavenged.